This invention relates to an amplification circuit and, more particularly, to an amplification circuit for a CCD (Charge Coupled Device), for example.
A circuit, as shown in FIG. 22, has been used as the amplification circuit for the CCD. A signal charge is transferred by the CCD to an output diffusion layer (which is expressed structurally equivalently in the form of a diode D), and is converted to the form of a signal voltage by a capacitor C1 which consists of the junction capacity of the output diffusion layer and a parasitic capacity of a reset MOSFET Q1 and a driver MOSFET Q2 of a source-follower circuit.
This signal voltage is outputted through the source-follower circuit which consists of the amplification MOSFET Q2 and a load MOSFET Q3. Such amplification circuit is referred to as an FDA (Floating Diffusion Amplifier). The MOSFET Q1 rests the signal charge retained in the capacitor Cl to a reference voltage VR with a reset pulse .phi.R when the signal voltage corresponding to the aforementioned signal charge is amplified and outputted or in other words, before a next signal charge is transferred.
Such FDA is described, for example, on page 64 of "CCD Camera Technique", published on Nov. 3, 1986.
The sensitivity of the aforementioned FDA is given by the product (As/C1) of the capacity value of the capacitor Cl including the parasitic capacity in the reset MOSFET Q1 and in the amplification MOSFET Q2 and the gain (As&lt;1) of the source-follower circuit. The improvement in the gain As is limited by the relation of As&lt;1, and the improvement in the sensitivity of the FDA depends on how much the capacity value of the capacitor C1 can be reduced. In the conventional FDA, therefore, great efforts have been made how to miniaturize the diode D1 and the amplification MOSFET in order to reduce the capacity value of the capacitor C1.
If the size of the amplification MOSFET Q2 is reduced, however, the output current essentially becomes smaller, and there occurs a contradictory problem that the load driving capacity of a post stage circuit is lost. Thus, it has been customary to cascade a plurality of source-follower circuits so as to make up for insufficiency of the driving capacity of the initial stage circuit.